Abstract

The long-standing problem of the Oosterhoff dichotomy is studied within
the framework of up-to-date convective pulsating models and synthetic
horizontal-branch (HB) computations. It is shown that the observed
properties of RR Lyrae-rich Galactic globular clusters can be put in
agreement with theoretical prescriptions for a constant original helium
(Y(MS) is approximately 0.23), on condition that in Oosterhoff type I
clusters the transition between ab- and c-type RR Lyrae variables occurs
near the blue edge for fundamental mode of pulsation, whereas in
Oosterhoff type II it is near the red edge for first-overtone mode.
Since type I clusters have redder HB morphology than type II systems,
such an evidence should support the suggestion that the RRab/RRc
transition depends on the evolutionary history of the variables, as was
suggested early on by van Albada & Baker (1973). The analysis of the
'Oosterhoff-intermediate' clusters in the Large Magellanic Cloud
strengthens this hypothesis still more. In fact, they turn out to
represent the observational counterpart of a predicted class of clusters
where, as a consequence of evolutionary track morphology and hysteresis
in the pulsation as well, the mean period of ab-type variables shoud
vary from Oosterhoff type I to Oosterhoff type II values. On these
grounds, both the Oosterhoff groups in the Galaxy and the
Oosterhoff-intermediate clusters in the large Magellanic Cloud could be
explained within the same theoretical scenario, the different
pulsational properties being caused by a combination of HB morphology
with metallicity.

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